Microglia isolated from newborn rat brains and cultured for 7 days, actively extrude protons in exchange for potassium ions creating a gradient of excess protons and a deficit of potassium ions in close proximity to the outer surface of the cell membrane. These concentration gradients, measured with an ion selective self-referencing electrode, dissipate over a distance of 10mm away from the cell and are activated by high concentrations of extracellular potassium ions or protons. Pharmacological and immunohisto-chemical studies revealed that an H+/K+-ATPase is the major generator of these gradients. Neither the Na+/K+-ATPase nor the inward rectifier potassium channel make significant contributions to the generation of these gradients. At 5mM extracellular potassium concentration, a gradient of -9.43+ 4.2 mM (n=48) was recorded dissipating over a distance of 10mm from the cell membrane. While the transporter activity could be blocked by Omeprazole (10mM) and by the specific H+/K+-ATP ase blocker SCH28080 (1mM), it was insensitive to ouabain and strophantidine. The Kd of the glial transporter for K ions is an order of magnitude higher (3.7 mM) than that of the epithelial H+/K+-ATPase. This is a first report of an H+/K+ transporter in microglial cells with Kd in the physio-logical range of [K+]out. Implications of the H+/K+-ATPase on potassium homeostasis in microglia under high extracellular potassium and low pH, as found at the site of brain injury, are discussed.